The battery fire in a grounded Boeing 787 jet in Boston on Jan. 7 was caused by short circuiting in a single cell, according to the National Transportation Safety Board, which released a public docket into the event on Thursday.
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However, two months after the incident, the NTSB says its investigation remains ongoing.
The NTSB has been working to determine the cause of the short circuiting and analyzing the “special conditions” granted to Boeing by the Federal Aviation Administration that allowed for the eventual certification of the lithium ion battery.
Boeing (BA) said it is reviewing the 500-page NTSB report.
This is a “positive step in the progress toward completing the investigation of the Jan. 7 event in Boston,” a Boeing spokesman said. “The Boeing team has worked tirelessly in support of the NTSB to help develop an understanding of the event and will continue to do so.”
NTSB Chairman Deborah Hersman last month said that evidence “strongly suggests” the Boston Logan event initiated in cell number 6 of the 787 battery auxiliary unit located in the belly of the plane. The charred battery components indicate the temperature inside the battery case exceeded 500 degrees.
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The NTSB has been working to find the cause of the short circuiting, though it has ruled out both mechanical impact damage to the battery and external short circuiting. Possible causes could be the method and delivery of cell charges, contamination or electric folds in the assembly of the cells and battery and the battery’s design, including the physical separation of the cells, their electrical interconnections and thermal isolation from each other.
The Japan Airlines fire two months ago was the start of a string of Dreamliner problems that eventually led to an emergency landing in Tokyo and triggered an airworthiness directive that has grounded all 50 in-service 787s.
During the incident at Boston Logan, smoke was discovered by cleaning personnel in the cabin of the aircraft, which was parked at a gate.
The NTSB continues to look into the certification process of the 787, which was granted nine “special conditions” by the Federal Aviation Administration related to the use of lithium-ion batteries.
“Of particular interest to our investigators is understanding how these special conditions relate to the failure modes and the outcome that we saw in this event: smoke and fire,” Hersman said last month at a press event.
Boeing studied possible failures that could occur within the battery during the certification process, including the likelihood of a failure occurring and the effects that failure could have on the battery. To do this, Boeing conducted several tests, including one that essentially short circuited a battery cell.
While Boeing said those tests showed “no evidence of cell to cell propagation or fire in the battery,” the NTSB said cell to cell propagation that resulted in a “cascading manner and a fire” resulted when cell No. 6 short circuited on Jan. 7.
The Chicago jet maker also determined at the same time that there was a less than one in every 10 million flight hour chance that the 787 battery would smoke. However, there were two critical battery events on two separate aircraft just two weeks apart on the 787 fleet with fewer than 100,000 flight hours resulting in smoke.
As part of the NTSB investigation this month, it tested of some of the batteries that were removed from the 787 fleet from the field and examined the safety certification process used by both the FAA and Boeing for the 787 battery design and determine why the hazards involved in the Japan Airlines incident were not mitigated.
The NTSB said it will hold a forum and a hearing in April to provide additional information to advance the investigation.